In case where the surface of a metal substrate is subjected to electrodeposition coating or a coating by paint application, for example, aqueous coating, solvent coating, powder coating or the like, the surface of the metal substrate is, before forming a coat, chemically treated to forma chemical conversion film for the purpose of improving the performance, such as corrosion resistance, adhesion to a coating film and the like.
Heretofore, as the chemical conversion treatment agent for the chemical conversion treatment, widely used are a chromate-type chemical conversion treatment agent and a zinc phosphate-type chemical conversion treatment agent. However, as containing chromium, the chromate-type chemical conversion treatment agent involves a risk of environmental load; and as containing a phosphate ion, the zinc phosphate-type chemical conversion treatment agent involves a risk of river and ocean nourishment.
Consequently, a chemical conversion treatment agent containing at least one of zirconium, titanium and hafnium has become specifically noted as a chemical conversion treatment agent substitute for those chromate-type chemical conversion treatment agent and zinc phosphate-type chemical conversion treatment agent.
For example, PTL 1 describes a chemical conversion treatment agent that comprises at least one selected from a group consisting of zirconium, titanium and hafnium, fluorine, and an amino group-containing silane coupling agent.
PTL 2 describes a chemical conversion treatment agent comprising at least one selected from a group consisting of zirconium, titanium and hafnium, fluorine, and a water-soluble resin, wherein the water-soluble resin is a polyvinylamine resin and/or a polyallylamine.
PTL 3 describes a chemical conversion treatment agent comprising at least one selected from a group consisting of zirconium, titanium and hafnium, fluorine, an adhesion imparting agent, and a chemical conversion reaction accelerator, wherein the adhesion imparting agent is at least one of a specific metal ion (A), an alkaline earth metal ion (B), a Periodic Table Group 3 metal ion (C), a copper ion (D), a silicon-containing compound (E), a water-soluble resin such as a polyallylamine resin or the like (F), an amino group-containing water-soluble epoxy resin (G), and a silane coupling agent or the like (H). As the silane coupling agent, there is mentioned an amino group-containing amino silane coupling agent.
The role of coating is mainly for esthetic purposes and protection. Depending on the intended use of products and parts, a suitable coating system is employed. For example, for products that require high-level corrosion resistance such as automobile bodies, electrodeposition coating is employed for corrosion-resistant primer coating. Electrodeposition coating is generally employed for so-called corrosion-resistant primer coating of which the main function is for corrosion resistance and adhesion enhancement. A design of a coating film excellent in adhesion and corrosive substance-blocking capability, typically with an epoxy resin or the like, is made.
On the other hand, a coating by paint application is employed for many products, for which a wide variety of coating materials are used. However, some coating materials are inferior to corrosion-resistant primers of typically electrodeposition paint in point of adhesion and corrosive factor-blocking capability. Specifically, an epoxy resin is rarely used as a top-coating paint from the viewpoint of weather resistance, and therefore the adhesion is often not good. To some products, a film thickness of more than 30 μm may be given, which, however, would bring about a problem of coating film stripping owing to stress.